[RADIOCARBON, VOL. 34, No. 2, 1992, P. 255-262] RECENT AND HISTORICAL SOLAR PROTON EVENTS M. A. SHEA and D. F. SMART Space Physics Division, Geophysics Directorate/Phillips Laboratory, Hanscom Air Force Base Bedford, Massachusetts 01731-5000 USA ABSTRACT. A study of the solar proton event data between 1954 and 1986 indicates that the large fluence events at the Earth are usually associated with a sequence of solar activity and related geomagnetic storms. This association appears to be useful to infer the occurrence of major fluence proton events extending back to 1934, albeit in a non-homogeneous man- ner. We discuss the possibility of identifying major solar proton events prior to 1934, using geomagnetic records as a proxy. INTRODUCTION Solar-flare-generated protons impacting the top of the atmosphere account for some of the total radiocarbon inventory. Lingenfelter and Ramaty (1970) have estimated that the solar flare production during the 19th solar cycle was between 6% and 14% of the solar-cycle averaged galactic cosmic-ray production rate. We assume that the source of these protons is associated, in some way, with the occurrence of a solar flare, although a unique acceleration process has identified. not been Solar-flare accelerated particles travel along the interplanetary magnetic field and impact the Earth's atmosphere principally in the polar regions. Occasional)y, the Sun accelerates particles into the GeV energy range; these particles can penetrate the geomagnetic shieldingg to lower latitudes and, hence, are more effective to 14C production. Approximately half of the energy of the solar flare is in the interplanetary shock that travels to the Earth's orbit 1 or 2 days after the flare. Under favorable circumstances, particularly during a sequence of solar activity, y g the shock re-accelerates the ambient solar particle population, thus modifying the observed particle spectrum from what was released into the interplanetary medium during the original flare process. When the interplanetary shock impacts the Earth's magnetosphere, the structure of the magnetosphere is altered. Often, depending on the direction of the interplanetary magnetic field vectors, a major geomagnetic storm will occur. The current systems involved in the geomagnetic storm can substantially lower the geomagnetic cutoff rigidity (FlUckiger, Smart & Shea 1986. The occurrence of mid- and low-latitude aurorae is evidence of a major expansion of the Earth's polar caps, which allows solar particles to penetrate much lower in latitude than during quiescent geomagnetic conditions. Figure 1 represents the major solar- flare emissions and the relative time intervals that these emissions take to travel to the orbit of the Earth. Using data acquired by both spacecraft and ground-based techniques, we have assembled a list of over 200 significant solar proton events that have reached the Earth between 1954 and 1986. We would like to suggest a method of identifying some of the major proton events over the past century, using a combination of solar flare, solar p article and geomagnetic records. SOLAR PROTON DATA BASE Although the first directly measured solar proton event occurred in 1942, it was not until the International Geophysical Year in 1957-1958 that solar proton events could be routinely identified. Even then, the identification y of solar proton events was limited tot the mayor events that could be 255 ee '5 aa 4` amt` Jib GGSS' 4 e` `Ci`aa m 5 CCs4 V 4.., ..CC 'o1664tiim 4t DS S' a ,o ,;:::ttOo cY GAG'15 ,F tt %o` S tee,, 45 t` ,a5 p5 S¢,,' tv$J `` fSS' t` s PrrD BOG S¢, '' 'bb 11 3y o JS t * ,,,, 0 '' p{{ J`` ,,'' ,,,,5 pG Ioo `' V' tSJ S°°, GSSS ,,qaa 0t 5 o,tt55',S `` `b4 o`i 00 o 4 I ` S c,'11, o 5000' S O. ,tii ,,55' 55`° e++ 'a ` 155 aa . ''55 ,,`oo ' {{{'' 001- p ,,o 4 ` vtt' OO oo54 osssBoa `` o f O SAO QJJJJJJ,, .pea ° o 's to'' oo o . O oo ` S,SS ot ,11, , 5 5e, I o1{`G aO a CC o , eee' or° ooyyyy 5°5 o`` ' 45, t° o,, Jp54aa5 `aa Qua 4 0t4 Ooo oao 'so s ° %% tito c, , ti5555 aoo f$ boy' '1OO ,`b yr % 5 5'''' SS SS, .100 S d t' ,,t O oa5 ra ss cc:r 4 s ,,,oo aa t\c' ,,,, 'S4oo ,1 opt 4, °°°' i''°°°°555° sGe, 444 , , SS' of 4i , s 5 GG 4 Recent and Historical Solar Proton Events 257 F CYCLE 19 CYCLE 20 CYCLE 21 I-A----CYCLE 22---i YEAR 8 FREQUENCY Wz6 W O 4 W m 2 J 2 0 I I I I T-1 r f Ill-n 1955 1960 1965 1970 1975 1980 1985 1990 YEAR Fig. 2. Relativistic solar proton events observed since 1955. Top - the smoothed sunspot number; Bottom - the number of high energy solar proton events each year. 200r CYCLE 19 CYCLE 20 w W 20 L o W 20 m a 15 a w 15 o Q 10 m 50 >. 10 m 5 w z 5 I 5 10 I jI1 YEARS FROM SUNSPOT MINIMUM 5 10 YEARS FROM SUNSPOT MINIMUM 200r 200 m m W CYCLE 21 W m m 0 0 Z 150 Xx x 0 x x A A CYCLE 19 X - CYCLE 20 x X x 0-CYCLE 21 I 5 10 I 5 10 YEARS 15 20 FROM SUNSPOT MINIMUM NUMBER OF PROTON EVENTS PER YEAR Fig. 3. The number of significant discrete solar proton events for each 12-month period after solar minimum and the 12-month mean sunspot number for the corresponding period (histograms) for solar cycles 19-21. A plot of the number of proton events per year vs. the yearly average sunspot number is shown in the lower right hand section. 258 M. A. Shea and D. F. Smart Fig. 4. Summation of significant discrete solar proton events for cycles 19-21 (.) and the corresponding 12-month average sunspot number (histogram). The data are organized in 12-month periods, beginning with the month after sunspot minimum, as defined by the number. 5 10 statistically smoothed sunspot YEARS FROM SUNSPOT MINIMUM the distribution In combining these results for the three solar cycles into one figure, we obtain events shown in Figure 4. From this, we conclude that the majority of significant solar proton occurs from the 2nd through the 10th years of the solar cycle. SOLAR PROTON FLUENCE more Although most solar particle events are measured in terms of peak proton flux, the fluence is 14C solar appropriate for production. Feynman et al. (1990) show that the fluence for events in cycles 19-21 all fit in one continuous log-normal distribution. Shea and Smart (1990a) indicate that events it is not always possible to identify the fluence on an event-by-event basis, because many flux occur in episodes of activity. In these cases, the fluence is determined by summing the when throughout the entire period. An example of a major episode of activity was August 1972, the proton a series of flares from one solar region, as it traversed the solar disk, produced one-half fluence above 10 MeV for the entire 11-year solar cycle. EVENTS IN SOLAR CYCLE 22 The present solar cycle, which started in October 1986, has been unexpectedly active, with respect to the generation of major solar proton events in both the energy content, flux and fluence. This reinforces the suggestion that we and other researchers have made that the first two solar cycles of the space era may be atypical cycles rather than normal cycles. This suggestion was first published by Goswami et al. (1988), who analyzed the spectral characteristics of solar proton events responsible for the generation of radio nuclei. These authors concluded that the spectral characteristics observed in the 20th and 21st solar cycles were different than observed in the 19th solar cycle. Shea and Smart (1990b) also suggested that the relativistic solar proton events of the 22nd solar cycle were similar to those in the 17-19th cycles with large fluxes and long durations; the relativistic proton events in the 20th and 21st solar cycles were generally smaller events of very short duration. Figure 5 illustrates the difference in duration of two relativistic solar proton events. On 7 May 1978 was the largest event of either the 20th or 21st solar cycle; the high energy solar proton flux, as measured at the Kerguelen Island neutron monitor, lasted for dust over two hours. On 29 September 1989 was the largest event of the 22nd solar cycle, to date, and is comparable to the events in November 1960, as well as events measured by ionization chambers prior to 1955. Recent and Historical Solar Proton Events 259 Kerguelen Island 290 240 -I 190 29 September 1989 140- 90- 40- 7 May 1978 -10 Hour Fig. 5. The duration of the largest relativistic solar proton event of the 22nd solar cycle (29 September 1989) compared with the largest event of the 20th and 21st solar cycles (7 May 1978). The Kerguelen Island neutron monitor S, is located at 49.35° 70.27° E and measures the cosmic radiation above -500 MeV. The high-energy solar proton flux was enhanced for about a day. Obviously, the fluence, which is proportional to the area under the respective curves, was considerably greater for the 29 September 1989 event. One of the conclusions in Shea and Smart (1990a) was that solar proton events occur in episodes of activity. This usually happens when one solar-active region produces a series of major flares with associated proton emission, as the active region rotates with the sun from the east to west limb.